Vitamin D is a fat-soluble steroid pro-hormone. Of the two major forms Ergocalciferol (D2) and Cholecalciferol (D3), only vitamin D3 is synthesized by the body.

Main vitamin D sources are: UV exposure from sun which leads to Cholecalciferol production in the upper layers of the skin, foods such as fish, shellfish, mushrooms, vitamin D fortified foods (e.g. milk, juice) and supplements. Approximately 10 – 20 % of vitamin D is supplied through nutritional intake.

Vitamin D is stored in adipose tissue and enters the circulation bound to vitamin D binding protein (VDBP) and albumin.In the liver, vitamin D is hydroxylated to give 25(OH)D which also circulates as a complex with VDBP. A small proportion of the 25(OH)D is further hydroxylated in the kidney, under direct regulation by parathyroid hormone and ionised calcium levels, to form the biologically-active calcitropic hormone 1,25-dihydroxyvitamin D [1,25(OH)2D]. Vitamin D plays a major role in the calcium and phosphorus homeostasis.

Vitamin D deficiency is a cause of hyperparathyroidism and diseases related to impaired bone metabolism (e.g. rickets, osteoporosis, and osteomalacia).